In the medical field, ultrasound diagnostic apparatuses are utilized for various kinds of diagnosis and treatment because they can be used to noninvasively examine the internal structure or blood flow state of a patient. When using an ultrasound diagnostic apparatus, the operator brings an ultrasound probe that includes transducers (piezoelectric transducers) at a distal end thereof into contact with the body surface of a subject and transmits ultrasound into the body of the subject, and then receives reflected waves that arise due to a mismatch of acoustic impedances inside the subject with the transducers of the ultrasound probe. An ultrasound image is generated based on reception signals that are obtained in this manner.
In recent years, a technique referred to as “harmonic imaging” is being developed that utilizes nonlinear components that are detected in very small amounts during propagation of an acoustic wave. The velocity of an ultrasound wave that propagates through a substance has a property such that a portion in which the sound pressure is high travels faster and a portion in which the sound pressure is low travels slower. Therefore, even when the transmitted ultrasound is a sinusoidal wave consisting of fundamental wave components, distortion gradually arises during the course of propagation, and consequently a higher harmonic wave having nonlinear components becomes included therein. Harmonic imaging that utilizes such kind of higher harmonic components are broadly divided into tissue harmonic imaging (THI) and contrast harmonic imaging (CHI) depending on the kind of signal that is visualized. THI is used for imaging higher harmonic components generated from tissue when ultrasound propagates through the tissue. CHI is used for imaging higher harmonic components generated when a contrast agent including microbubbles (bubbles) for ultrasound is administered to a subject and the bubbles resonate and disintegrate.
In harmonic imaging, fundamental wave components and higher harmonic components are separated by filtering or waveform calculation to extract the higher harmonic components. For example, in a technique referred to as “amplitude modulation”, the ultrasound that is transmitted is transmitted three times so that the relative sound pressures thereof are ½, 1, and ½, and by subtracting the total of two reception signals in which the transmission sound pressure is ½ from a reception signal in which the transmission sound pressure is 1, the fundamental wave components can be removed and the higher harmonic components can be extracted. Various techniques other than amplitude modulation are also being developed, such as a phase inversion technique and a technique that combines amplitude modulation and phase inversion.
As described above, in harmonic imaging, plural sets of transmission and reception may be performed to extract higher harmonic components, the plural sets being performed in different sound pressures. A method is also used that sets a voltage applied to the transducers to different values to set the transmission sound pressure to different values of 1 and ½. However, it is difficult to set an applied voltage to different values with high accuracy. Therefore, the sound pressure is adjusted by actuating all of the transducers when the transmission sound pressure is 1, and actuating half of the transducers when the transmission sound pressure is ½. In the case of setting the transmission sound pressure to ½, in order to match the transmission aperture with the transmission aperture in a case where the transmission sound pressure is 1, control is performed so as to actuate odd-numbered or even-numbered transducers (that is, every second channel).
However, there has been the problem that, due to crosstalk, that is, leaking in of transmission signals from channels for which transducer are driven, transmission pulses are generated even from channels that are not transmitting, that is, channels in which a transducer is not driven. In some cases, due to transmission from channels that are not transmitting, the relative sound pressure that is actually transmitted to a subject exceeds ½. Accordingly, fundamental wave components cannot be cancelled out by the aforementioned amplitude modulation technique or phase inversion technique, and tissue or bubble components linger and cause a decrease in the S/N ratio or a deterioration in the image quality.
Therefore, an ultrasound diagnostic apparatus has been proposed in which crosstalk is avoided by performing transmission at a low voltage from channels that are not transmitting. According to a related art, because a previously set transmission pulse is transmitted from channels that are not transmitting, a fundamental waveform is cancelled by subsequently subtracting those components.
Instead of the above described technology, an ultrasound diagnostic apparatus is desired that enables more accurate transmission sound pressure control by ensuring transmission pulses are not generated due to crosstalk in channels that are not transmitting.